Kirk Douglas plays Rolf Pedersen, a Norwegian physics professor, who, though originally content to wait out the war, is soon pulled into the struggle by local resistance leader Knut Straud (based on Knut Haukelid, portrayed by Richard Harris).

They are both smuggled to Britain to have microfilmed plans of the hydroelectric plant examined, and then return to Norway to plan a commando raid. When a force of Royal Engineers, who were to carry it out, are all killed, Pedersen and Straud lead a small force of saboteurs into the plant. The raid is successful, but the Germans quickly repair the equipment.

The Germans then plan to ship steel drums of heavy water to Germany. Pedersen and Straud sabotage a ferry carrying the drums, and it sinks in the deepest part of a fjord.

Besides this sequence, the raids (Operations Grouse, Freshman, and Gunnerside) and the final attack are depicted in location filming, in which snowy Norwegian locations serve as a backdrop for the plot.

Knut Haukelid wrote a memoir of the attack called Skis Against the Atom published in 1954.[3] John Drummond wrote a novel based on the same story called But for These Men. Both books formed the basis of the screenplay.

Scenes early in the film when the main characters escape from occupied Norway were filmed around Poole and Hamworthy in Dorset with the former Channel Islands ship TSS Roebuck playing the role of the hijacked steamer. SF Ammonia was used to represent the train ferry SF Hydro in the final fjord scenes.

It was amongst the 15 most popular films at the British box office in 1966.[9]

Stephen Boyd later sued Mann for half a million dollars when funding for the initial project fell through in early 1964. "I missed out on four good roles and plenty of money when he signed me without financial backing and then dropped the project," said Boyd later. "He asked me again later but I'd made other commitments so Kirk Douglas and Richard Harris made it under another title."[10]

Ray Mears made a documentary called The Real Heroes of Telemark. Despite mainly sticking to the factual evidence, some scenes in the documentary, like the film, were partly dramatised, focusing more on the survival skills involved in the operation.

The same story was also covered in the 1948 Franco-Norwegian film Kampen om tungtvannet (La bataille de l'eau lourde — "The battle for heavy water"). Quite faithful to the real events, it even had many of the original Norwegian commandos starring as themselves.